1. Field of the Invention
The present invention relates to a resin which is improved in flame retardance by a simple method. Particularly, it relates to a flame-retardant resin composition which is further improved in thermal stability and flexibility and can give an appearance of being free from bleeding and the flexibility of which is not lowered by heat history, and an electric wire coated with said composition.
2. Discussion of Related Art
Up to this time, rubbers, polyvinyl chloride, polyethylene, polypropylene and nylon have been used as electric wire coating materials. Among these materials, polyvinyl chloride in particular has been frequently used by virture of its flame retardance and mechanical strengths. Recently, as the environment in which these coating materials are used has become severe, the characteristics required of the coating materials have also become severe. That is, the coating materials have been required to have not only excellent heat resistance, electrical characteristics and flame retardance but also be capable of forming a coating thin enough to attain space saving.
Although fluororesins and crosslinked polyethylene satisfy these requirements, they are poor in their capability of forming a thin coating. Further, fluororesins are expensive, thus being unsatisfactory.
An increasing interest has been taken in polyethylene terephthalate and polybutylene terephthalate, because they exhibit a good capability of forming a thin coating and are excellent in mechanical strengths (such as flexibility and wear resistance), heat resistance and electrical characteristics. However, they are insufficient in flame retardance.
Various methods for imparting flame retardance to these resins have been known, which are broadly classified into addition type and reaction type. The methods of the former type can simply impart flame retardance to the resins by using antimony hydroxide, antimony trioxide, boron compounds, bromine compounds or phosphorus compounds. However, the methods have disadvantages in that the added flame retardant bleeds from the surface of the resin when subjected to prolonged exposure or in a heated environment to, thereby inflicting damage to the appearance of the resin and that the added flame retardant lowers the flexibility of the resin. The methods of the latter type are characterized by using a reactive flame-retardant monomer, oligomer or polymer prepared by introducing an epoxy or vinyl group or an ester-forming functional group into a monomer, oligomer or polymer containing bromine or phosphorus atoms and comprise incorporating such a flame-retardant group in a molecular skeleton or in a polymer molecule by the reaction of polymers with each other. These reactive flame retardants cause no bleeding and impart flexibility to the resin, so that the methods of reaction type is free from the disadvantages of those of the addition type, thus being advantageous in this regard. However, the incorporation of a flame-retardant monomer in a polymer skeleton by an ordinary process results in a low degree of polymerization, so that a complicated operation is required for the incorporation with a satisfactory degree of polymerization. Although the addition of a reactive flame-retardant oligomer or polymer is free from the disadvantage as described above, it has another disadvantage in that the interaction between a base polymer and such an oligomer or polymer occurs during kneading to cause a lowering in the molecular weight or mechanical properties. Particularly, this disadvantage is significant for a polyester or polyamide.
Further, the above-mentioned polyalkylene terephthalate resins exhibit significant lowering in the flexibility and mechanical strengths such as impact resistance by heat history including thermal treatment after coating and thermal condition in service owing to their crystalline nature, so that the use thereof around a heat source or in an environment in danger of heat buildup must be avoided. Thus, the use of polyalkylene terephthalate is considerably limited.
With the purpose of overcoming the disadvantage as described above, there have been made attempts of adding an elastomer or an amorphous polymer to the resin to thereby lower the crystallinity thereof as much as possible and attempts of partially crosslinking the resin to thereby maintain its mechanical strengths.
Although the former attempts give a slightly recognizable improvement, the resin composition improved according to the former attempts still contains crystalline resin matrix, so that the composition is not tolerable to prolonged heat history and exhibits mechanical characteristics (such as wear resistance) lowered by a reduced content of crystalline resin in the composition.
Although the latter attempts give a slightly recognizable improvement in the stability of mechanical characteristics, due to the crosslinking they are disadvantageous in that the crosslinked resin exhibits lowered flexibility and that the crosslinking is difficult to control and brings about a remarkable lowering in the processability.